The present invention relates to signal processing, and more particularly to a method of encoded video decoding to improve the appearance of television pictures by eliminating cross color and dot crawl effects.
The image portion of an encoded video signal has a chrominance signal added to a luminance signal, the luminance signal describing the brightness of the picture for every point, or pixel, on a television screen and the chrominance signal describing the hue and saturation of the corresponding colors. In the NTSC system the chrominance signal is the combination of two signals that are referred to as the "I" and "Q" signals. These signals are derived from the Red, Green and Blue signals produced by television cameras and other video sources and represent color in a rectangular coordinate system. The values of I and Q individually may be either positive or negative, describing points in any of the four quadrants of the color coordinate system. The I and Q signals are used to modulate quadrature components of a subcarrier signal that are added together to form a composite chrominance signal. By sampling the composite chrominance signal at the peaks and zeros of the subcarrier signal the amplitudes and polarities of the I and Q signals may be recovered. This straight forward approach for decoding the color components is complicated by the luminance signal.
The final encoded video signal is the combination of the composite chrominance signal with the luminance signal. Therefore when sampling the encoded video signal the samples become combinations of both luminance and I or Q. The problem is the separation of the luminance and the chrominance. The modulated chrominance signal occupies the upper portion of the luminance passband, and cross color and cross luminance (dot crawl) errors occur due to errors in the separation process. For example a pattern of black and white diagonal stripes at the frequency of the subcarrier signal is essentially a chrominance signal so that luminance patterns approximating that condition produce cross color errors. When the encoded video signal is separated, energy is lost by the luminance signal and gained by the chrominance signal, resulting in loss of luminance detail as well as a disturbing rainbow pattern. Dot crawl occurs when edges between brightly colored objects produce energy outside the decoders chrominance passband so that, when the decoder separates the encoded video signal, energy is lost from the chrominance signal and gained by the luminance signal, resulting in a change in color as well as dot crawl. The greater problem in either case is not the loss, but the gain in energy, i.e., the lack of sharpness within a striped pattern is far less obvious than the moving rainbow that obviously does not belong there, and a slight color change at an edge is far less noticeable than even a small magnitude of dot crawl.
Therefore what is desired is a decoding scheme that reduces cross color and dot crawl errors while preserving detail and edge sharpness to the greatest extent possible, with no loss of detail in areas of the television picture that are free from cross color and dot crawl errors.